The factors that determine the energy density of a redox flow battery are the concentration of the redox ions in solution, the cell potential and the number of electrons transferred during discharge per mole of active redox ions. In the case of the all-vanadium redox flow cell, the maximum vanadium ion concentration that can be employed for wide temperature range operation is typically 2 M or less. This concentration represents the solubility limit of the V(II) and/or V(III) ions in the sulphuric acid supporting electrolyte at temperatures below 5° C. and the stability of the V(V) ions at temperatures above 40° C.
The use of a vanadium (IV) bromide solution in both half-cells of an all vanadium bromide redox flow cell was described in Australian patent application PS1921 “Vanadium Bromide Redox Flow Battery” and PCT Application, PCT/GB2003/001757 “Metal Bromide Redox Flow Cell”. This system involves the use of a solution of 0.1 to 5 M vanadium (IV) bromide in HBr/HCl mixtures in both the positive and negative half-cell electrolytes, thereby overcoming the problem of cross-contamination of the two half-cell solutions. The higher solubility of V(II) and V(III) bromide in this systems allows much higher energy densities to be achieved compared with the vanadium sulphate based redox flow cell.
In Australian patent application PS1921 “Vanadium Bromide Redox Flow Battery” and PCT Application, PCT/GB2003/001757 “Metal Bromide Redox Flow Cell”, a V(IV) bromide solution is used in equal volumes in both half-cells. In these patents, it was proposed that during the initial charge cycle, the V(IV) ions are first oxidised to V(V), followed by the oxidation of the Br− to Br3− or Br2Cl− in the positive half-cell, while V(IV) is reduced by a 2-electron process to V2+ in the negative half-cell. Subsequent charge-discharge cycling involves the one-electron V2+/V3+ oxidation-reduction reaction in the negative half-cell and the Br−/Br3− redox reactions in the positive half-cell.
Further investigations by the inventors, have however revealed that V(IV) is not oxidised to V(V) to any appreciable extent in the presence of the high bromide ion concentration required to stabilise the bromine produced at the positive electrode, so that during the initial charge cycle, the positive electrolyte must undergo oxidation of two moles of Br− ions for every mole of V(IV) reduced to V2+ at the negative electrode. Similarly, on discharge, the V2+ ions are oxidised to V3+ by a one-electron reduction process, so only half of the formed bromine is converted back to the original Br− form during the discharge cycle. This means that the positive half-cell electrolyte always contains excess bromine or the relatively unstable Br3− or Br2Cl− species that could give rise to bromine gas emission problems during operation of the vanadium bromide battery. Furthermore, the presence of excess bromine in the positive half-cell electrolyte increases the corrosive properties of this solution, reducing the life of the cell components.
It is therefore desirable to alter the composition of the feed electrolyte for the vanadium halide redox flow cell to avoid the production of excess bromine during cell operation. It is also desirable to alter the electrolyte production process to avoid generation of excess bromine during electrolyte preparation.
The inventors have also discovered that by further adjusting the composition of the initial feed solution for both half-cells of the vanadium halide redox cell, it is possible to halve the volume of the positive half-cell solution and still achieve the same capacity during cycling. This would allow a 25% decrease in the volume and weight of the electrolytes, thereby increasing the energy density and specific energy of the vanadium halide system by up to 25%, providing an important benefit for mobile applications in particular.
The inventors have further found that by complexing, immobilising or gelling the vanadium halide cell electrolytes, it is possible to stabilise the bromine produced, so that a greater fraction of the bromide ions can be oxidised during charging in the positive half-cell electrolyte without significant bromine loss.